There are a variety of optical systems in which high precision mirrors are used. Examples include ultraviolet lithography condenser mirrors, reflective laser scanner mirrors, remote sensing for homeland security, exo-atmospheric reflecting optics, high energy infrared (IR) laser systems, solar power concentrators, systems in which low scattering is needed for visible radiation, and a variety of other applications.
One technique for making precision metal mirrors is to take a substrate of an aluminum alloy, such as aluminum 6061-T6, and to carry out single point diamond turning (DPT) of a surface on the substrate, which then serves as the reflective surface. This alloy is lightweight, is easily machined by DPT, and has good long-term stability. However, the aluminum alloy contains alloy elements such as zinc, chromium and iron, which leave defects or artifacts after DPT that effectively limit the surface finish achievable with DPT. Such a surface finish provides adequately low scatter for many applications in which the reflected radiation of interest has a relatively long wavelength, such as a wavelength greater than 3.0 microns. At shorter wavelengths, however, such as the wavelength of visible radiation, such a surface finish produces a level of scatter which is too high for many applications.
Scattering is a result of corrosion, as well as a higher than desirable surface finish which exposes sites at which the alloy material may be exposed, and this results in a decrease system throughput and in reducing laser induced damage threshold (LIDT) performance. Nickel plating, and more recently an aluminum plating process called AlumiPlate® (AlumiPlate Incorporated, Minneapolis, Minn.), which is an electroplating process for depositing aluminum layers, have overcome some of the issues relating to corrosion, but not all of the issues that enable the preparation of high performance mirrors that can be used over a wide wavelength range.
Nickel plated finished optics have demonstrated enhanced corrosion resistance when exposed to harsh environment tests such as salt fog and extended humidity and have been observed to have enhanced laser damage threshold performance. However, there is an inherent mismatch between the coefficient of thermal expansion (CTE) for the aluminum alloy substrate and the CTE for the plated nickel layer. Consequently, and due to the thickness of the nickel layer, there can be a bimetallic effect between the substrate and nickel layer, which causes bending of the mirror surface in the operational temperature range of the mirror. Such bending is undesirable in the context of a high precision mirror surface because it changes optical characteristics of the mirror surface. In order to keep such bimetallic bending to an acceptable level over normal temperature variations, the plated nickel has to be a thin and uniform layer. As such, the nickel plated finished optics cannot be considered for applications having large operational temperature ranges, for example, between about −70° C. and about 60° C.
An alternative technique uses electro-deposited high purity aluminum plating instead of the nickel plating, in order to avoid the thermal mismatch. This process creates a hard aluminum oxide outer layer on the electro-deposited aluminum layer, and this oxide layer damages the diamond tool during cutting. In addition, this high purity aluminum is very soft, and tends to build up on the diamond tool during cutting, which makes the DPT operation difficult. Also, the soft, aluminum surface is easily scratched, and difficult to clean.
Yet another technique includes polishing the aluminum substrate. This is difficult because of the softness of the aluminum, although some techniques have demonstrated surface finishes of 10 Å RMS. However, bi-directional reflective distribution function (BRDF) scatter testing shows that the resulting aspheric surface on the polished 6061-T6 aluminum layer effectively performs like a 60 Å RMS surface finish, because the surface peak-to-valley variations remain very high as a result of impurities. As a result, such polishing techniques do not provide a significant improvement over the unpolished aluminum substrate.